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Table are it was 48iference bet

Beginning of January to the 3d of May, and after that, the Emersions out of Jupiter's Shadow, to December, for the several Days, Hours, Minutes, and Seconds, when they happen, as you here observe in their respective Columns.

Euphrof. I see them all ; and take Notice, that at the End of the Table, he subjoins an Example of its Use, which is the very Thing I want to see explained.

Cleon. That you shall instantly; as thus ; fuppose you was in some particular Place on the oth Day of next October, and there, with a good reflecting Telescope, you observed the first Satellite of Jupiter emerging out of the Shadow at 44 Minutes and 22 Seconds paft 10 o'Clock at Night, by a good Watch, that shews Seconds; then you would take the Ephemeris, to see what Time the same Phænomenon happened at the Royal Observatory at Greenwich, (to which the Numbers in the Table are adapted ;) and you will find against Otober the gth, that it was 48 Minutes and 42 Seconds after 8 at Night. Now the Difference between the Times of Obfervation at Greenwich, and the Place you are supposed to be in, is 1 Hour, 55 Minutes, and 40 Seconds, which, converted into Degrees and Minutes, in the Equator, will make 28 Degrees, and 5 Minutes, allowing for every Minute of Time 15 Minutes of a Degree, as before mentioned.

Euphrof. So that these 28 Degrees and 5 Minutes are what you call the Difference of Longitude of those two Places; but Mr. White says, this is the Longitude of the Place of Observation to the East of the Britis Observatory: But why does he say to the East, Cleonicus ? this I do not clearly understand.

Cleon. This you will easily apprehend, my Euphrosyne, if you consider, that the Diurnal Motion of the Earth is Eastward, and therefore, at whatever Moment of Time the Eclipse is seen at a Place 15 Degrees Eastward, it must necessarily be one Hour later than the same appears at the Observatory at Greenwich; and if the Place were Westward 15 Degrees, then it would happen 1 Hour fooner than at the Observatory ; but of this Affair you will be more particularly convinced when we come to the Use of the Globes,

before moros. So the Dite says, 'Eaft of

7 22 36 101 Immersions

MAY

A Table of the Eclipses of Jupiter's first Satellite,

reduced to apparent Time, 1757 Immersions | Immersions Immersions Emersions JANUARY FEBRUARY APRIL June d. h. m. 1. d. h. m. f.d. h. m. 1. d. h. m. f.

2 15 13 41 26 11 42 1622 8 36 2812 18 31 57 14 941 11 28 6 10 5324 3 5 14 14 13 0 151 16 4 8 43 MARCH

25 21 34 116 728 33

27 16 2 47 18 i 5650 9 17 3.49

29 10 31 3319 20 25 31 II II 31 19 2 0 39 31

21 14 53 271 13 5 58 49 3 19 8 io

23 9 21 461 15 0 26 301 5 13 36 53 Immersions 125 3 50 g 16 18 54 12 7 8 5 37

26 22 18 331 18 13 21 56 9 2 34 271 1 5 0 10 28 16 46 531 20 7 49 41 10 21

3 171 2 23 28 59 30 11 15 13 22 2 17 28 12 15

12 15 32 7 Emersions JULY 23 20 45 1514 25 15 13 9 16 4

20 5 50 Emersions 127 9 41 417 22

6 14 34 28 2 5 43 35 29 4 9 219 17 27

8 9 3 014 011 57 130 22 37 11 21

3 31 42 5 18 40 21

22 0 19 7 13 8 46|| FEBRUARY/23 6 25 25 0 54 4

16 28 511 9 7 37 131 | Immersions 126 10 22 8115 10 57 2311 2 5 411

1 17 5 14 28 13 51 53117 5 26 12 20 34 171 3 11 33 18 30 8 20 54 18 23 54 37 14 15 2 53 5 6 1 27

20 18 23 416 9 31 26

APRIL 7 0 29 37

22 12 51 3218 4 0 0 18 57 51 Immersions

24 7 19 5719 22 23 371 13 26 6

26 I 48 22 22 16 57 14 sa 231 1 2 49 55 27 20 16 4523 11 25 50 14 2

1 2 21 18 47 29 14 45 825 5 54 38 15 20

31 4 15 47 39 31 9 13 29/27 0 23 221 $17 15 20) 6 10 16 31

JUNE 28 18 52 71 19 9 47 591 8 4 45 2412

130 13 20 551

Emersions 21 4 16 331 9 23 14 18_E

AUGUST 22 22 45 611 17 43 12 2 3 41 511 124 17 13 401'3 12 12 41 3 22 10 18! Emersions

15 6 40 56 5 16 38 461 7 49 44 117 I 9 53 7 11 7 4 3 2 18 41 18 19 38 509 5 35 23 4 20 47 381

7 391 0 3 406 15 16 36

20 14

A Table of the Eclipses of Yupiter's first Satellite,

reduced to apparent Time, 1757. Enerions Emersions | Emersions | Immersions

August SEPTEMBER OCTOBER DECEMBER d. h. m. 1. d. h. m. f. d. h. m. f.d. h. m. f. $94; 3417

7 12 i 42 5 19 50 2011 23 41 31 29 6 31 17 14 19 36 13 18 8 39

a I 0 24 9 Il 22 43 3:11

8 48 42 15 12 36 151 2,12 19 29 4811 3 17 4817 7

13 59 812 21 46 51 19 31 18 § 28 28 14 16 15 53 20 19 58 51

57 45116 10 44 5122 14 26 23

27 418 5 13 49 24 8 15 56 20 19 23 42 48 26 3 21 33

25 3.21 18 11 46

4 54 54 23 12 40 37 29 16 16 36 25 23 24 11'25 7 9 27 31 10 44 5! 28 17 53 30'27 I 38 11

10 1: 2: 49 28 20 6 55 1:1 10 440

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SEPTEVIER. Ocrosse 30 14 35 40

Emertors Emerficas Conjunction 1 : 335i

of the Sun and | ; :; ; ; : 6;2.3 Jupiter, NoIsi 3:24.4 i 2116 vezber 2ift

The Times of the Eclipses in this Table are adapted to the seridian of the Royal Observatory near Leeds, and afford an excellent Method of dilJourering the Longitude, or Difference of Meridians, between that and any other place whatsoever, which I inul illustrate by an Example.

Suzpuè ca the oth Day of 027ober, the Time of the Emrtion of riter's firt Satellite, be observed by a Telescope at 4 lin. 22 Sec. paft 10 at Night; I thard by the Table, that the Time of this EmerHtion will happen at the Britijh Observatory, the uume Visht, 45 Viia. 42 Sec. paft 8: The Difterence of the Times is 1 Hour, 55 Min. 40 Sec. which converted into Deg. and Min, of the Equator, wili make 28 Deg. 55 Min. the Longitude off che Place of Obiervation, to the East of the Bri21,6 Obervatory.

Euphrof. Well, I suppose by this Time you are tired of talking so much about the Longitude, to one of our Sex, who are so seldom employed in putting in Practice any of the great Discoveries of the Philosophers; but as we have naturally a Curiosity of Enquiry into every Thing that we hear of that is of a public or wonderful Nature, we are oftentimes solicitous to be satisfied about Things that do not so immediately concern us. Of this I shall give you a farther Instance by a Query or two concerning Saturn, his Moons, and Ring. I see they move, after the same Manner with those of Jupiter, about their Primary; but I observe this System of Moons has not the same Position with respect to the Plane of the Orrery as those of Jupiter have; but as the Time is now far advanced, I shall beg a more particular Account of what relates to Saturn and his System the next Opportunity we have for Converse on these Subjects.

DIALOGUE VIII.
The Use of the Orrery continued.

Cleonicus.
T Remember you took Notice, that Saturn's Moons were

not alike pofited with those of Jupiter, in regard to the Ecliptic. And it is true, they are not; for those of Jupiter are parallel to that; but those of Saturn are inclined thereto in an Angle of about 31 Degrees; as is also the Plane of his Ring.

Euphrof. Then, I perceive, that the Shadow of Saturn's Moons, cast on the Pasteboard behind them, will not appear to move backward and forward in a right Line, like those of Jupiter.

Cleon. You rightly observe, they cannot appear so to move to an Eye placed upon the Earth ; as you will easily see by the Experiment; for having placed the Pasteboard properly behind this Planet, and taken all the Candles but one out of the Room--lay your Hand on the Winch, and put them in Motion, and then you see on the Pasteboard, that each respective Satellite describes an Orbit of an oval Figure, and what the Geometers properly call an Ellipfis, one Part of which lies above, and the other below the Planet, and its Ring in the Center,

Euphrof. It greatly delights me to observe these curious Appearances; and now I see, in Fact, how Things are in Nature perforined, and brought about: I see the Reafon why the Moons, while they describe the remote Part of their Orbit, appear direct in Motion, and retrograde, while they describe that Part next to us :- I see likewise, that the Shadow of the Ring, in like Manner, is not circular, but elliptical, including the Body of Saturn, very much like what formerly appeared in the Heavens through the Telescope.

Cleon. It must undoubtedly be pleasant to see the wonderful Machinery of Nature thus represented in Epitome, and yet, at the same Time, so perfectly; but there is one Thing which you have not yet adverted to, and that is, that Saturn and his whole System, and the Motion about the Sun, observe a Paralism of Position; or in other Words, the Planet, his Ring, and Satellites, always respect the same Part of the Heavens in every part of their Orbits; and this you will easily perceive, if you attend to it but a very short Time; but as this is a curious Phænomenon, I shall represent it to you in the Orrery, by Means of the Lamp in the Place of the Sun, and the Pasteboard, connected with the Stem of the Planet ; so as always to be behind it, by which Means the Shadow of the Ring will cast a Shadow upon the Pasteboard, and thereby the several Phases of the Ring will plainly appear, as they are observed through a Telescope in the Heavens during the several Parts of his long Period *.

Euphrof. This will be a delightful Spectacle, indeed; but, I fear, it will cost you a good deal of Time and Trouble; since the Motion of this Planet is so very now, even in the Orrery itself.

Cleon. I shall think no Time or Trouble too much, or ill-spent, to inform the Mind of my dear Euphrosyne. Beside that, we need only observe the Phases of the Ring through one fourth Part of its Period; and seven Years, you know, will soon be over in the Orrery; but it will be necessary, in the first Place, to bring this Planet to that Part of its Orbit where the Plane of its Ring is most

of

of a very, mall in the Pelstem of this th

* The Reader will here cast his Eye upon Plate XX, where these Phases are delineated.

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